Jenkins works fine with dozens to hundreds of slaves per master, thousands of jobs, and tens of thousands of builds per day. Jenkins isn't perfect for everything, but it can be a very useful task runner. People have adapted it to do almost everything these days, from database maintenances to builds to restarting infrastructure. Given the almost unlimited triggering and reporting, you could use it to build many ETL apps as well (not saying that's a good idea).

As much as Jenkins sucks sometimes, it's what many big shops use for many things. There is tons of community around it. It works, it's battle-tested, and it's been that way for years. Anything else will have a hard time catching up for as many use-cases as Jenkins currently solves for people.

If you hate a part of it, fix it and open a PR :)